Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.t33f..06r&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #T33F-06
Physics
8121 Dynamics: Convection Currents, And Mantle Plumes, 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8125 Evolution Of The Earth (0325), 8147 Planetary Interiors (5430, 5724, 6024)
Scientific paper
We have explored through deformation experiments and numerical modeling, the nature of liquid metal-silicate interaction under dynamic conditions and the implications for metal migration and core-mantle interaction. We report the results of a series of deformation experiments on a sample of partially molten Kernouve H6 chondrite (T = 900 to 1050°C) aimed at determining the siderophile concentrations in both Fe-S-Ni-O quench and Fe-Ni metal as a function of degree of melting, and to provide insight into the melt segregation mechanism(s). The relationship between melt fraction and porosity is used to compare the migration rate of liquid metal driven by buoyancy pressure gradients with a theoretical model of melt segregation in a deforming porous medium that takes into account the coupling between volume strain (dilatancy) and shear stress. For buoyancy driven porous flow, highest transport velocities occur at highest porosities, implying the fastest flow velocities will carry Fe-rich liquid metal with low sulfur contents, preferentially enriched in incompatible HSEs. Predicted characteristic timescales of liquid metal transport due to buoyancy effects (diapirism and porous flow) for a 100 km-sized planetesimal are contrasted with shear-induced segregation velocities set up in response to external perturbations via impacts, an important process during the final stages of planetary accretion. A feature of our analysis is that liquid metal segregated previously into a planetary core by buoyancy instabilities (e.g. porous flow or a raining mechanism), might be drawn locally back into the silicate lower mantle by pressure gradients linked to surface impacts providing a physical mechanism for return flow of siderophile elements across the CMB. During the latter (impact) stage of accretion that is likely to favor shear-enhanced melt flow, existing core material may be transported locally back into the silicate proto-mantle, providing a way of imparting a late-stage siderophile imprint that is analogous to the late veneer. Our geochemical modeling for the Earth has shown that the approximately chondritic relative abundances of PGEs in the upper mantle can be explained by about 0.1-0.2% admixture of bulk core liquid, without the need to invoke a late chondritic veneer. In the current Earth, impacts are no longer a driving force for deformation, but downwelling of cold mantle may drive deformation along portions of the CMB. We have undertaken a set of zeroth order calculations using loading of the CMB by cold downwelling mantle as the main deformation mechanism. The calculations derive estimates of the pressure changes and subsequent fluid flow rates relevant to a deformable poroviscoelastic matrix as a function of applied shearing stress and are aimed at quantifying the effects of shear-enhanced deformation in porous layers on liquid metal return flow into D'' at a range of length and timescales.
Humayun Munir
Petford Nick
Rushmer Tracy
Yuen Dave
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